Debonding failure of sandwich-composite cryogenic fuel tank with internal core pressure

A summary of the failure analyses and testing that were conducted to determine the cause of the X-33 liquid-hydrogen tank failure is presented. Ply-level stress analyses were conducted to explain the formation of microcracks in the plies of the inner and outer facesheet laminates of the honeycomb sandwich walls of the tank under known thermal and mechanical loads. The microcracks allowed the ingression of liquid- and gaseous-hydrogen and gaseous-nitrogen purge gas that produced higher than expected sandwich core pressures in the tank. Single cantilever beam tests were used to determine the toughness of the interface between the facesheets and honeycomb core. Fracture mechanics analyses were developed to determine strain-energy release rates for known foreign object debris shapes and sizes and known and statistically possible core internal pressures. The fracture mechanics analyses were validated by comparing with results of blowoff tests that were fabricated from undamaged tank sandwich material. Strain-energy release rates from the validated analyses were then compared with known and statistically possible values of toughness determined from the single cantilever beam tests. These analyses and tests were then used to substantiate a scenario for failure of the X-33 liquid-hydrogen tank that includes microcracking of the inner facesheets and ensuing ingression of hydrogen and nitrogen, a low bondline strength and toughness, and the presence of foreign object debris.